1954.
While it is refered to as a pre 27, it was simply called the 357 Magnum at the time as it was the only one made. In 1957 S&W began assigning model numbers and it became the model 27.
Value depends heavily on condition, finish and barrel length with nickel and 3.5" barrel bringing the most money.
For a standard 6.5" blued in 90% with original grips I would guess about $800+.
Jim